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Hawaiian Islands Synoptic Discussion and Guidance

607
FXHW60 PHFO 240122
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST WED APR 23 2014

.SYNOPSIS...
BREEZY AND GUSTY TRADE WINDS WILL CONTINUE INTO SATURDAY AS A STRONG
RIDGE HOLDS SEVERAL HUNDRED MILES NORTH OF THE ISLANDS. MODEST
RAINFALL WILL REMAIN FOCUSED ACROSS WINDWARD SLOPES...THOUGH SOME
SHOWERS WILL BE BRIEFLY CARRIED TO LEEWARD AREAS ON THE TRADES. LOOK
FOR AN INCREASE IN SHOWER ACTIVITY AS A BAND OF MOISTURE MOVES
THROUGH THURSDAY NIGHT AND EARLY FRIDAY. TRADES WILL WEAKEN SUNDAY
AND MONDAY.

&&

.DISCUSSION...
THE ISLANDS WILL REMAIN LOCKED IN A BREEZY AND GUSTY TRADE WIND FLOW
INTO SATURDAY. SURFACE RIDGING CENTERED ABOUT 700 MILES NORTH OF THE
STATE BETWEEN 30N AND 35N WILL BE DRIVING THE FRESH TO STRONG
TRADES. IN SPITE OF THE RIDGE BEING CURRENTLY SPLIT BY A DISSIPATING
FRONT NORTH OF THE ISLANDS...THE PRESSURE GRADIENT OVER THE ISLANDS
REMAINS A RATHER STRONG WITH LITTLE CHANGE SINCE YESTERDAY. THE
AFTERNOON LIHUE SOUNDING SHOWS THE INVERSION UP NEAR 8000 FT DUE TO
A MOISTURE BAND SKIRTING JUST NORTH OF THE GARDEN ISLE. THE HILO
SOUNDING CAPTURED THE INVERSION AT A LOWER 5500 FT WHILE RECENT
AIRCRAFT OUT OF HNL HAVE DETECTED THE INVERSION DOWN NEAR 3500 FT.
GIVEN THE LOW INVERSION AWAY FROM KAUAI...THERE WILL LIKELY BE A
GOOD AMOUNT OF GUSTS IN THE 30/S WITH LOCALIZED GUSTS OVER 40 MPH
OVER AND TO THE LEE OF ISLAND TERRAIN TONIGHT.

THE GUSTY TRADES COULD BE A NOTCH STRONGER ON THURSDAY AND FRIDAY. A
1029 MB SURFACE HIGH WILL PASS ALONG THE RIDGE DURING THIS TIME.
THE GFS AND ECMWF SHOW 850 MB WINDS...ROUGHLY AROUND 5000 FT...
CRANKING UP FROM THE CURRENT 20 TO 25 KT INTO THE 30 TO 35 KT RANGE.
STRONG MID LEVEL RIDGING SHOULD MAINTAIN A LOW INVERSION HEIGHT...
BUT A COMPLICATING FACTOR WILL BE A SHOWER BAND...CURRENTLY NORTH OF
KAUAI...EXPECTED TO MOVE THROUGH THURSDAY NIGHT AND FRIDAY. IF THE
INVERSION STAYS LOW AND STRONG ON EITHER SIDE OF THE BAND...WE COULD
SEE POCKETS OF ADVISORY LEVEL WIND GUSTS OF 50 MPH OVER AND DOWNWIND
OF TERRAIN. CONDITIONS APPEAR TO BE BORDER LINE AT THIS TIME...BUT
WE WILL BE CLOSELY MONITORING NEW MODEL DATA FOR SIGNS OF STRONGER
WINDS. GUSTY TRADES WILL BE DOWN ONLY SLIGHTLY ON SATURDAY.

MODEST RAINFALL WILL BE FOCUSED ACROSS WINDWARD SLOPES WITH AN
UPTICK IN SHOWER ACTIVITY EXPECTED THURSDAY NIGHT AND FRIDAY. A
SHALLOW BAND OF MOISTURE CURRENTLY ALONG 23N WILL SKIRT JUST NORTH
OF KAUAI THROUGH THE DAY ON THURSDAY THEN PASS OVER THE ISLANDS
THURSDAY NIGHT AND EARLY FRIDAY. ASIDE FROM THIS FEATURE...NO OTHER
ORGANIZED AREAS OF MOISTURE ARE CURRENTLY FORECAST THROUGH AT LEAST
THE WEEKEND.

TRADE WINDS WILL FINALLY BACK OFF ON SUNDAY. AN UPPER LEVEL TROUGH
WILL DEVELOP OVER THE CENTRAL NORTH PACIFIC...ERODING THE SURFACE
RIDGE NORTH OF THE STATE AND SENDING A FRONT SOUTHWARD. TRADE WINDS
WILL SLOWLY DECLINE ON SUNDAY AND BOTTOM OUT AT MODERATE STRENGTH BY
MONDAY. THE FRONT WILL DISSIPATE BEFORE REACHING THE ISLANDS...BUT
WE COULD SEE SOME SHALLOW REMNANT MOISTURE RIDE IN DURING THE MIDDLE
OF THE WEEK AS TRADE WINDS GRADUALLY BUILD.

&&

.AVIATION...
GUSTY TRADE WINDS CONTINUE...REQUIRING AIRMET TANGO FOR MECHANICAL
TURBULENCE TO THE LEE OF THE MOUNTAINS. ANTICIPATE THIS AIRMET TO
REMAIN POSTED THROUGH AT LEAST THURSDAY.

VFR CONDITIONS ARE EXPECTED TO PREVAIL...HOWEVER LOW CLOUDS AND
SHOWERS BEING CARRIED IN ON THE TRADES WILL BRING PASSING MVFR
CONDITIONS...POSSIBLY IFR CONDITIONS...TO MAINLY WINDWARD AND
MOUNTAIN AREAS. WINDS ARE STRONG ENOUGH THAT SOME OF THE SHOWERS MAY
BLOW TO LEEWARD AREAS OF THE SMALLER ISLANDS.

&&

.MARINE...
TRADE WINDS WILL REMAIN IN THE FRESH TO STRONG RANGE INTO
SATURDAY...WITH THE STRONGEST WINDS EXPECTED THURSDAY AND FRIDAY.
THE SMALL CRAFT ADVISORY /SCA/ HAS BEEN EXPANDED TO COVER ALL WATERS
IN ANTICIPATION OF THE STRENGTHENING WINDS IN ADDITION TO INCREASING
COMBINED SEAS. THE SCA IS IN EFFECT THROUGH FRIDAY AFTERNOON BUT
WILL PROBABLY BE SCALED DOWN TO THE TYPICALLY WINDY WATERS AROUND
THE BIG ISLAND AND MAUI ON SATURDAY NIGHT OR SUNDAY. WINDS WILL BE
NORTHEASTERLY AND AT THEIR MINIMUM LATE SUNDAY AND MONDAY...THEN
INCREASE AGAIN DURING THE MIDDLE OF NEXT WEEK.

WITH THE ISLANDS SQUARELY PLACED IN A BELT OF TRADE WINDS...WINDWARD
BUOYS HAVE BEEN REGISTERING SEAS OF 7 TO 8 FT AT 8 TO 9 SECONDS
TODAY. SOME INCREASE IN WIND-DRIVEN SEAS IS EXPECTED THROUGH
FRIDAY...AND THIS WILL RESULT IN ROUGH SURF NEAR THE ADVISORY LEVEL
FOR EAST FACING SHORES. THE INCREASING SEAS ALSO CONTRIBUTE TO THE
NEED FOR THE EXPANDED SCA. EXPECT A DECLINE LATE SATURDAY AND SUNDAY.

A NORTHWEST SWELL WILL CONTINUE TO BUILD INTO TONIGHT. THE LATEST
READINGS AT BUOY 51101 SHOW THAT THE SWELL IS BIGGER THAN WAVEWATCH
GUIDANCE AND WILL RESULT IN SIGNIFICANT WAVE HEIGHTS JUST BELOW THE
NORTH SHORE ADVISORY CRITERION ON THE SMALLER ISLANDS LATE TONIGHT
AND THURSDAY. THE SWELL WILL DROP FRIDAY AND SATURDAY WITH A SMALL
REINFORCING NORTHWEST SWELL DUE LATE SUNDAY INTO TUESDAY.

NOT TO BE LEFT OUT...SOUTH FACING SHORES ARE SEEING SOME SWELL
ENERGY FROM THE SOUTHWEST. THIS SWELL WILL FADE ON FRIDAY WITH LOW
ODDS FOR SOME SMALL SOUTH-SOUTHEAST SWELL THIS WEEKEND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR ALL WATERS.

&&

$$

DISCUSSION/MARINE...WROE
AVIATION...MBALLARD

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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